template<typename T> // insert after node a
void DLL<T>::insertAfter( DLLNode<T>& a, const T& el) {
  DLLNode<T> *n = new DLLNode<T>(el, a->next, a);
  a->next = n;
  if( a == tail) tail = n; else n->next->prev = n;
}

template<typename T> // insert before node b
void DLL<T>::insertBefore( DLLNode<T>& b, const T& el) {
  DLLNode<T> *n = new DLLNode<T>(el, b, b->prev);
  b->prev = n;
  if( b == head) head = n; else n->prev->next = n;
}